1. Field of the Invention
The invention relates broadly to the investigation of geological formations. More particularly, this invention relates to in situ methods of determining the viscosity of heavy oils using nuclear magnetic resonance (NMR) techniques.
2. Description of Related Art
Most of the world's oil reservoirs contain heavy and viscous hydrocarbons which are difficult and costly to produce. Heavy oil viscosity is one of the few criteria available to assess production economics by helping predict if cold production will yield economic production rates, or if thermal processes will be required to reduce the oil viscosity to achieve the required production rates. If cold production is selected, viscosity is again used to help determine whether vertical or horizontal wells should be used. Viscosity data are also used to adjust cold production exploitation strategies if the production rates are significantly lower than expected.
The use of NMR techniques has been known to provide a good correlation between viscosity and NMR relaxation time for relatively light oils. However, such techniques fail for highly viscous oils (heavy oils).
More particularly, NMR relaxation time of bulk fluids is sensitive to the viscosity and temperature due to the dependence of rotational and translational correlation times of fluids. Presently in the petroleum industry, there are three widely used correlations between oil viscosity and the NMR logarithmic mean of the spin-spin relaxation time distribution:
                              T                      2            ⁢                                                  ⁢            LM                          =                              1200                          η              0.9                                ⁢                      (                          Straley              ⁢                              -                            ⁢              Kleinberg              ⁢                              -                            ⁢              Vinegar              ⁢                                                          ⁢              correlation                        )                                              (        1        )                                          T                      2            ⁢                                                  ⁢            LM                          =                  7.13          ⁢                      T            η                    ⁢                      (                          Zega              ⁢                              -                            ⁢              Zhange              ⁢                                                                                ⁢                                                                              ⁢              correlation                        )                                              (        2        )                                          T                      2            ⁢                                                  ⁢            LM                          =                  9.56          ⁢                      T            η                    ⁢                      (                          Lo              ⁢                                                          ⁢              correlation                        )                                              (        3        )            where η is the viscosity of the oil in centipoise (cp), T is the temperature in degrees Kelvin, and T2LM is the logarithmic mean of the T2 distribution in milliseconds (msec). Unfortunately, as can be seen from FIG. 1 which plots the viscosity values measured in a lab (using a capillary viscometer) for heavy oil (HO) samples collected from different locations against the viscosities predicted by the correlations set forth above (using a 2 MHz Maran Ultra NMR instrument available from Oxford Instruments plc of Abingdon, Oxon, United Kingdom), none of the above expressions provided a good correlation.